We get the number we got in step 3 (20) and we ADD it to the number we got in step 2 (69):20 + 69 = 89

Now we take the number we got in step 4 (89) and work out how much we have to add to round it up to the nearest 10. In order to round 89 up to the nearest 10 (90) we have to add... 1

Is this value of 1 the same as the last (rightmost) digit in our code - the one we ignored in steps 1 and 3 (that's our checksum)?

YES, the checksum in our UPC was also 1 so the UPC is valid!

The rightmost digit in a UPC is a checksum, because it provides some insurance that all the other numbers are right by performing the above calculation on them. The system is not foolproof, but if any number is wrong then you will typically get a wrong checksum.